1. Field of the Invention
The present invention relates to a solid-state image sensor which is a semiconductor device adapted to detect a physical quantity distribution, and to an image capturing apparatus using a solid-state image sensor. More particularly, the present invention relates to a mechanism of reading a physical quantity distribution output in the form of an analog electric signal from each of pixel cells which are sensitive to an electromagnetic wave such as light or radiation incident from the outside and which are arranged in the form of an array, and outputting the detected physical quantity distribution converted into the form of digital data to the outside.
2. Description of the Related Art
For example, in the technology of video devices, it is known to use a solid-state image sensor of a CCD (Charge Coupled Device) type or a MOS (Metal Oxide Semiconductor) or CMOS (Complementary Metal Oxide Semiconductor) type to detect physical quantity of light (which is one form of electromagnetic wave). In this type of solid-state image sensor, a physical quantity distribution is detected and converted into the form of an electric signal by pixel cells. Herein, the term “solid-state” is used to indicate that the device is made of semiconductor.
One type of solid-state image sensor is an active pixel sensor (also called a gain sensor) in which each pixel is configured such that a driving transistor for amplifying a pixel signal is provided in an pixel signal generator which generate the pixel signal corresponding to a signal charge generated by a charge generator. Most of MOS or CMOS-type solid-state image sensors use this configuration.
The CCD-type solid-state image sensor (hereinafter referred to as a CCD solid-state image sensor or more simply as a CCD image sensor) needs a special production process and needs a plurality of power supply voltage in operation. Besides, it is necessary to use a plurality of peripheral ICs (semiconductor Integrated Circuits) in operation, and thus a system is very complicated.
In contrast, the CMOS-type solid-state image sensor (hereafter referred to as a CMOS solid-state image sensor or a CMOS image sensor) has many advantages over the CCD image sensor. First, this type of solid-state image sensor can be produced using a production process very similar to that widely used over the world to produce CMOS semiconductor integrated circuits. Second, it can be driven using a single power supply. Third, analog circuits and logic circuits can be formed on the same single chip by using the CMOS production process, and thus it is possible to reduce the number of peripheral ICs.
Because of the above-described great advantages, much attention has been recently paid to CMOS image sensors.
In the active pixel sensor such as the CMOS image sensor, a pixel signal is read pixel by pixel from a plurality of pixels arranged in the form of an array such that a particular pixel is selected by specifying its address, and a signal is read from the selected pixel. In this sense, the active pixel sensor is an address control type solid-state image sensor.
For example, in a X-Y address control active pixel sensor in which pixel cells are arranged in the form of an array, each pixel has an amplification capability implemented by an amplifier such as a floating diffusion amplifier using an active element of a MOS structure (MOS transistor), whereby a signal charge (photoelectrons) accumulated in a photodiode serving as a photoelectric conversion element is amplified by the active element and the amplified signal is output as image information from a pixel array unit to a signal processing circuit connected to the pixel array unit.
In the operation, accumulation of the signal charge corresponding to incident light is performed line by line or pixel by pixel, and a current or voltage signal corresponding to the accumulated signal charge is sequentially read from the pixels by an address control scheme.
In most CCD image sensors, an output circuit is configured in the form of a 1-channel output circuit using a floating diffusion amplifier. In contrast, in CMOS image sensors, as described above, each pixel has an amplifier such as a floating diffusion amplifier, and pixel signals of the respective pixels are read from the pixel array unit, for example, on a row-by-row basis. More specifically, rows in the pixel array unit are selected one by one, and all pixels located in a selected row are simultaneously read in parallel. This signal outputting scheme is called a column parallel outputting scheme.
The parallel outputting scheme is needed because the amplifier disposed in each pixel does not have high enough driving capability to output a pixel signal in a short period, and the data rate at which the pixel signal is read out from one pixel is reduced by using the parallel outputting scheme.
In some solid-state image sensors, an analog image signal output from a pixel array unit is converted into digital data by an analog-to-digital converter, and resultant digital data is output to the outside.
In some column parallel output image sensors, the conversion of the image signal from analog form into digital form is also performed. As for the signal output circuit for this purpose, many types have been proposed. In an example of a most advance type, one analog-to-digital converter is provided for each column to convert a pixel signal into digital data (see, for example, Japanese Unexamined Patent Application Publication No. 2005-323331).
As for the analog-to-digital converter, various types are known depending on the conversion speed, the conversion resolution, the circuit complexity. In one type known as a single-slope integration or a ramp signal comparison analog-to-digital converter, an analog pixel signal is compared with a ramp-shaped reference signal, and a time spent to complete the comparison operation is counted. Digital data corresponding to the pixel signal is acquired on the basis of the count value obtained at a point of time at which the comparison is completed. This analog-to-digital conversion technique is also employed in the Japanese Unexamined Patent Application Publication No. 2005-323331.